Composition Comprising Rare-earth Dielectric

ABSTRACT

Compositions comprising a single-phase rare-earth dielectric disposed on a substrate. Embodiments of the present invention provide the basis for high-K gate dielectrics in conventional integrated circuits and high-K buried dielectrics as part of a semiconductor-on-insulator wafer structure.

STATEMENT OF RELATED CASES

This application is a continuation of U.S. patent application Ser. No.11/253,525 filed on Oct. 19, 2005, which is a continuation-in-part ofco-pending U.S. patent application Ser. No. 11/025,693 filed on Dec. 28,2004, which claims priority of provisional patent application U.S. Ser.No. 60/533,378 filed on Dec. 29, 2003. Furthermore, this case is relatedto U.S. patent application Ser. No. 11/254,031, filed on Oct. 19, 2005,and incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to compositions for electronic devices andintegrated circuits.

BACKGROUND OF THE INVENTION

Silicon is the material of choice for the fabrication of ultra largescale integrated (“ULSI”) circuits. This is due, in large part, to thefact that silicon is the only semiconductor substrate material that canbe formed as large-diameter wafers with sufficiently low defect density.Silicon also forms a high-quality oxide (i.e., silicon dioxide), and thequality of the silicon/silicon dioxide interface is also quite high.

It is notable that germanium, while otherwise very desirable as amaterial for the fabrication of integrated circuits, has not achievedcommercial acceptance. This is due to its high cost and lowavailability, which is a direct consequence of the defect-relatedlow-yield of germanium substrates. This high-defect density results fromdifficulties with producing bulk germanium substrates.

But the baton might soon be passed from the silicon/silicon dioxidematerial system to other materials in order to satisfy industryrequirements for continued advancements in the performance of integratedcircuits and the removal of roadblocks to the continuation of Moore'sLaw.

More specifically, as node-scaling of ULSI circuits is pushed furtherinto the sub-micron regime, silicon dioxide becomes increasingly lessattractive as an interlayer dielectric material (e.g., gate oxide,etc.). In particular, as silicon-based interlayer dielectrics becomeextremely thin, fundamental problems arise, such as an increase inquantum mechanical tunneling current, a decrease in dielectric breakdownstrength, and a decrease in reliability. Furthermore, decreases in thethickness of silicon-based interlayer dielectrics in ULSI electronicswill result in increases in capacitance, which cause concomitantincreases in RC (interconnect) delays and cross talk. This adverselyimpacts device speed and power dissipation.

A further issue with the silicon/silicon dioxide material system is thatin conventional integrated-circuit processing, a rate-limiting andyield-limiting step is the production of a gate oxide of sufficientquality. In order to produce a high-quality gate oxide, thesemiconductor surface is subjected to high temperatures while undervacuum to desorb any native oxide that has formed during prior waferprocessing. Once this native oxide is desorbed, a new gate oxide isformed on the newly-cleared surface. As ULSI technology continues toscale further into the sub-micron regime, it will become increasinglydifficult and expensive to obtain the gate oxides required.

In order to address these problems, the semiconductor industry hasundertaken a search for alternative materials for use as an interlayerdielectric. Suitable alternative materials should exhibit:

i. a dielectric constant (K) higher than that of silicon dioxide;

ii. large conduction and valence band offsets with silicon;

iii. thermal stability and reliability;

iv. high-quality dielectric/semiconductor interface;

v. low impurity concentration; and

vi. manufacturability.

Single-crystal rare-earth dielectrics are an attractive choice forhigh-K dielectric materials. Unfortunately, these materials do notnaturally occur, nor can they be produced using prior-art growthtechniques. On the other hand, amorphous, poly-crystalline, ormulti-domain crystalline rare-earth oxides are possible to produce. Butthese rare-earth oxides are ill-suited for high-performance integratedcircuits since they do not exhibit some of the characteristics listedabove. In addition, the thickness to which many prior-art rare-earthdielectrics can be grown on silicon is limited.

Rare-earth dielectrics deposited using prior art techniques have notrealized single domain, single crystal stoichiometry that enables highperformance integrated circuit devices. The inherent limitation of mostprior art techniques is a lack of elemental control during deposition.These rare-earth dielectric deposition techniques typically involve theevaporation of constituent rare-earth oxide powders to deposit evaporanton the substrate. Due to the very high melting point of these oxides,coupled with a very low vapor pressure, e-beam evaporation has been themost commonly used technique in the prior art.

Rare-earth dielectrics comprising alkaline-oxide layers arecharacterized by a negligible conduction band offset relative tosilicon. In addition, alkaline-oxide layers on silicon are limited inthickness to approximately 10 monolayers, due to crystal relaxation anddefect formation. These alkaline-oxide films, therefore, provideinsufficient utility as interlayer dielectric layers in high-performancesilicon integrated circuits.

Continued scaling of ULSI into the sub-micron regime is also pushing thelimits of the silicon substrate itself. It is widely expected that thefuture of ULSI will be based on semiconductor-on-insulator wafers thatutilize fully-depleted field-effect transistors (FETs) formed in anultrathin, active layer of silicon. Fully-depleted electronic devicesbecome viable as the thickness of the active layer is reduced below 100nanometers (nm).

Currently available methods to produce silicon-on-insulator (“SOT”)wafers rely on wafer bonding of oxidized silicon wafers followed byremoval of most of one substrate the two substrates to form the activelayer. Although several variations of this technology exist, all areincapable of providing ultrathin active layers of sufficient quality forfully-depleted electronics. In addition, the interface quality andimpurity concentration of the buried oxide layers is insufficient tosupport high-performance integrated circuitry. Finally, the complexityof wafer bonding processes used to produce SOT wafers is quite high,which leads to high costs. For these reasons, among any others, theacceptance of SOI wafers by the semiconductor industry has been ratherlimited.

SUMMARY OF THE INVENTION

The present invention provides compositions having a single-phaserare-earth dielectric disposed on a substrate. As described in detaillater in this specification, single-phase morphology is characterized bya single-crystal, single-domain crystalline structure. The dielectric isdeposited via an epitaxy process.

In some embodiments, the basic dielectric-on-substrate composition formsthe basis for a semiconductor-on-insulator layer structure thatincorporates a single-phase semiconductor layer. The semiconductor layeris grown on the rare-earth dielectric via an epitaxy process. Thisstructure is analogous to an SOI wafer in the prior art; wherein therare-earth dielectric serves as the buried oxide layer and thesemiconductor layer is the upper, active layer of silicon.

In some further embodiments, one or more additional layers of dielectricmaterial and/or semiconductor material are added to thesemiconductor-on-insulator structure disclosed herein. For example, insome of these embodiments, an additional layer of single-phaserare-earth dielectric is grown on the semiconductor layer. Thisadditional layer can serve as a gate dielectric for a transistor.

In some other embodiments, the additional layers of dielectric andsemiconductor are periodically arranged. These various compositions canbe used, for example, in the formation of double-gate structures andother complex semiconductor-based devices and high-performanceintegrated circuits.

A distinguishing feature of the compositions disclosed herein is themorphology of the rare-earth dielectric(s) and, in some cases, themorphology of the semiconductor(s). In particular, these compositionswill include at least one layer of a rare-earth dielectric that exhibitssingle-phase morphology. Some compositions include a semiconductor thatalso exhibits single-phase morphology, which, in fact, is enabled by thepresence of the single-phase rare-earth dielectric.

The presence of single-phase materials in the compositions disclosedherein results in high-quality dielectric/semiconductor interfaces, suchas are required for high-performance devices and circuits. Furthermore,rare-earth dielectric layers that exhibit single-phase morphology, asdisclosed herein, do not suffer from a limitation on thickness, asexhibited in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a composition that includes a single-phase rare-earthdielectric on a substrate, in accordance with the illustrativeembodiment of the present invention.

FIG. 2 depicts a variation of the illustrative embodiment, wherein thecomposition of FIG. 1 is used to form a semiconductor-on-insulatorarrangement.

FIG. 3 depicts a variation of the illustrative embodiment, wherein anadditional single-phase rare-earth dielectric is disposed on thesemiconductor-on-insulator arrangement of FIG. 2.

FIG. 4 depicts a variation of the illustrative embodiment, wherein acomposition includes a periodic arrangement of a single-phase activelayer on a single-phase dielectric layer, repeated as desired.

FIG. 5 depicts a crystal structure diagram of an oxygen-vacancy-derivedfluorite derivative, according to the illustrative embodiment of thepresent invention.

FIG. 6 depicts a chart of the polymorphs of rare-earth oxides versustemperature and as a function of cation radius.

FIG. 7A depicts a crystal structure diagram of a composition comprisinga rare-earth oxide disposed on a substrate, in accordance with the priorart.

FIG. 7B depicts a crystal structure diagram of the composition of FIG.1.

FIG. 8A depicts a layer structure comprising energetically-favorablebonding sites for rare-earth oxide anions, according to the illustrativeembodiment of the present invention.

FIG. 8B depicts a layer structure comprising a miscut substrate thatenables single-phase crystalline growth of a rare-earth dielectric,according to an alternative embodiment of the present invention.

FIG. 8C depicts a layer structure comprising a layer of ytterbiummonoxide (YbO) that enables single-phase crystalline growth of abixbyite structure rare-earth dielectric, according to an alternativeembodiment of the present invention.

FIG. 8D depicts a layer structure comprising a superlattice ofalternating layers that enables single-phase crystalline growth of abixbyite structure rare-earth dielectric, according to an alternativeembodiment of the present invention.

FIG. 9A depicts a layer structure comprising an interface layer thatprovides a suitable surface for epitaxial growth of a non-polarsemiconductor on a rare-earth dielectric, according to the illustrativeembodiment of the present invention.

FIG. 9B depicts a layer structure comprising an interface that providesa suitable surface for epitaxial growth of a non-polar semiconductor ona rare-earth dielectric, according to an alternative embodiment of thepresent invention.

DETAILED DESCRIPTION

For clarity and ease of understanding, this Detailed Description isorganized into several sections. A brief introduction to the disclosurethat follows is provided by way of the following outline.

I. Definitions

This section provides explicit definitions for certain terms and phrasesused in this Specification, including the appended claims.

II. Overview

This section highlights the importance of “single-phase” morphology asit relates to embodiments of the present invention.

III. Compositions including a Single-Phase Rare-Earth Dielectric on aSubstrate

This section discloses the basic structure of compositions in accordancewith the invention.

IV. Crystal Structure of Rare-Earth Dielectrics

This section describes the crystal structure of rare-earth dielectricsin general, and, more particularly, the crystal structure that isdesired for rare-earth dielectrics that are suitable for use inconjunction with the present invention.

V. Considerations for Producing a Single-Phase Rare-Earth Dielectric

This section addresses important considerations in selecting and growingrare-earth dielectrics that are suitable for use in conjunction with thepresent invention.

A. Cation-radius Dependency of Rear-Earth Dielectrics

Rare-earth cations having radii larger than 0.93 angstroms areunsuitable for use in conjunction with the present invention. Suitablerear-earth cations are disclosed.

B. Avoidance of Multi-Domain Growth of Rare-Earth Dielectrics

Rare-earth dielectrics are typically polar. Growing polar rear-earthdielectrics on a non-polar substrate (such as silicon or germanium)usually results in multi-domain growth, which is unacceptable for use inconjunction with the present invention. As described herein, specifictechniques are employed to ensure single-domain growth.

1. Surfaces suitable for Growing Single-Phase Rare-Earth Dielectrics

In order to form single-phase rare-earth dielectrics on a non-polarsubstrate, such as a silicon wafer, the substrate surface must provideenergetically-favorable bonding sites. Several methods have been foundto provide a substrate surface that enables epitaxial growth ofrare-earth dielectrics having a suitable morphology. Compositions thatare produced via these methods are disclosed.

VI. Active-Layer Epitaxy on Rear-Earth Dielectrics

In order to form a semiconductor-on-insulator structure that is suitablefor high-performance FET devices, the “active” layer should have asingle-phase crystal structure. The optimal deposition surface forproducing a single-phase active layer (e.g., silicon or germanium) viaepitaxy is non-polar, since silicon and germanium are non-polarcrystals. But most rare-earth dielectrics typically comprise polarcrystals.

Several methods have been found to provide a rare-earth dielectricsurface that enables epitaxial growth of single-phase non-polarsemiconductors, such as silicon and germanium. Compositions that areformed using these methods are disclosed

I. DEFINITIONS

The following terms are defined for use in this Specification, includingthe appended claims:

Layer means a substantially-uniform thickness of a material covering asurface. A layer can be either continuous or discontinuous (i.e., havinggaps between regions of the material). For example, a layer cancompletely cover a surface, or be segmented into discrete regions, whichcollectively define the layer (i.e., regions formed using selective-areaepitaxy).

Monolithically-integrated means formed on the surface of the substrate,typically by depositing layers disposed on the surface.

Disposed on means “exists on” an underlying material or layer. Thislayer may comprise intermediate layers, such as transitional layers,necessary to ensure a suitable surface. For example, if a material isdescribed to be “disposed on a substrate,” this can mean either (1) thematerial is in intimate contact with the substrate; or (2) the materialis in contact with one or more transitional layers that reside on thesubstrate.

Single-crystal means a crystalline structure that comprisessubstantially only one type of unit-cell. A single-crystal layer,however, may exhibit some crystalline defects such as stacking faults,dislocations, or other commonly occurring crystalline defects.

Single-domain means a crystalline structure that comprises substantiallyonly one structure of unit-cell and substantially only one orientationof that unit cell. In other words, a single-domain crystal exhibits notwinning or anti-phase domains.

Single-phase means a crystalline structure that is both single-crystaland single-domain.

Substrate means the material on which deposited layers are formed.Exemplary substrates include, without limitation: bulk silicon wafers,in which a wafer comprises a homogeneous thickness of single-crystalsilicon; composite wafers, such as a silicon-on-insulator wafer thatcomprises a layer of silicon that is disposed on a layer of silicondioxide that is disposed on a bulk silicon handle wafer; or any othermaterial that serves as base layer upon which, or in which, devices areformed. Examples of such other materials that are suitable, as afunction of the application, for use as substrate layers and bulksubstrates include, without limitation, germanium, alumina,gallium-arsenide, indium-phosphide, silica, silicon dioxide,borosilicate glass, pyrex, and sapphire.

Miscut Substrate means a substrate which comprises a surface crystalstructure that is oriented at an angle to that associated with thecrystal structure of the substrate. For example, a 6.degree. miscut<100> silicon wafer comprises a <100> silicon wafer that has been cut atan angle to the <100> crystal orientation by 6.degree. toward anothermajor crystalline orientation, such as <110>. Typically, but notnecessarily, the miscut will be up to about 20 degrees. Unlessspecifically noted, the phrase “miscut substrate” includes miscut wafershaving any major crystal orientation. That is, a <111> wafer miscuttoward the <011> direction, a <100> wafer miscut toward the <110>direction, and a <011> wafer miscut toward the <001> direction.

Semiconductor-on-Insulator means a composition that comprises asingle-crystal semiconductor layer, a single-phase dielectric layer, anda substrate, wherein the dielectric layer is interposed between thesemiconductor layer and the substrate. This structure is reminiscent ofprior-art silicon-on-insulator (“SOI”) compositions, which typicallyinclude a single-crystal silicon substrate, a non-single-phasedielectric layer (e.g., amorphous silicon dioxide, etc.) and asingle-crystal silicon semiconductor layer. Several importantdistinctions betweens prior-art SOI wafers and the inventivesemiconductor-on-insulator compositions are that:

Semiconductor-on-insulator compositions include a dielectric layer thathas a single-phase morphology, whereas SOI wafers do not. In fact, theinsulator layer of typical SOI wafers is not even single crystal.

Semiconductor-on-insulator compositions include a silicon, germanium, orsilicon-germanium “active” layer, whereas prior-art SOI wafers use asilicon active layer. In other words, exemplarysemiconductor-on-insulator compositions in accordance with the inventioninclude, without limitation: silicon-on-insulator,germanium-on-insulator, and silicon-germanium-on-insulator.

In some embodiments, the semiconductor-on-insulator compositions thatare disclosed herein include additional layers between the semiconductorlayer and the substrate.

II. OVERVIEW

The present invention provides compositions having a single-phase (i.e.,single crystal and single domain) rare-earth dielectric disposed on asubstrate. In some embodiments, this basic dielectric-on-substratecomposition forms the basis for a semiconductor-on-insulator structurethat adds a single-phase semiconductor layer on the rare-earthdielectric. In some further embodiments, the semiconductor-on-insulatorarrangement forms the basis for compositions that include additionallayers. Among other uses, the compositions disclosed herein are usefulfor forming high-performance integrated circuits. It is notable that thecompositions described herein include layers having a unique,single-phase morphology. In fact, these layers could not have been madeusing existing techniques, as they have been practiced in the prior art.

III. COMPOSITIONS COMPRISING A SINGLE-PHASE RARE-EARTH DIELECTRIC ON ASUBSTRATE

FIG. 1 depicts composition 100A in accordance with the illustrativeembodiment of the present invention. FIGS. 2-4 depict severalalternative embodiments, which incorporate and expand on composition100A.

Composition 100A comprises substrate 102 and single-phase rare-earthdielectric 104. Composition 100B, which is depicted in FIG. 2, is asemiconductor-on-insulator structure that includes composition 100A(i.e., substrate 102 and single-phase rare-earth dielectric 104) as wellas single-phase active layer 106.

In some embodiments, substrate 102 is a standard silicon wafer having a<100> crystal orientation, as is well-known to those skilled in the art.In some alternative embodiments, substrate 102 is a wafer other than<100> silicon. Other suitable substrate materials include, withoutlimitation, <111> silicon, <011> silicon, miscut <100> silicon, miscut<111> silicon, miscut <011> silicon, germanium, and miscut germanium.

Single-phase rare-earth dielectric 104 (hereinafter, dielectric 104) isepitaxially-grown on and monolithically-integrated with substrate 102.Among any other purposes, dielectric 104 provides a high-K dielectriclayer that electrically isolates active layer 106 from substrate 102. Insome embodiments, dielectric 104 comprises erbium oxide. Additionalmaterials suitable for use as dielectric 104 include, withoutlimitation:

Other rare-earth oxides, such as oxides of ytterbium, dysprosium,holmium, thulium, and lutetium;

Rare-earth nitrides, such as nitrides of erbium, ytterbium, dysprosium,holmium, thulium, and lutetium;

Rare-earth phosphides, such as phosphides of erbium, ytterbium,dysprosium, holmium, thulium, and lutetium; Typically, the thickness ofdielectric layer 104 is in the range of 0.5 to 5000 nanometers. In someembodiments, the thickness of dielectric layer 104 is preferably in therange of 1 to 10 nanometers.

Single-phase active layer 106 is epitaxially-grown on andmonolithically-integrated with dielectric layer 104. Active layer 106 isa layer of single-phase semiconductor suitable for formation ofhigh-performance integrated circuits. Exemplary semiconductors suitablefor use as active layer 106 include silicon, silicon carbide, germanium,and silicon-germanium. In some additional embodiments, active layer 106is a compound semiconductor, such as gallium arsenide, indium phosphide,and alloys of gallium arsenide and indium phosphide.

In the illustrative embodiment, active layer 106 has a thickness of 50nm; in some alternative embodiments, active layer 106 has a thicknessother than 50 nm.

Turning now to FIG. 3, composition 100C includes composition 100B aswell as an additional layer 104-2 of single-phase dielectric.

This additional layer of dielectric 104-2 comprises single-phasematerial that is epitaxially-grown on and monolithically-integrated withactive layer 106. Among any other purposes, dielectric 104-2 is usefulas an interlayer dielectric, gate dielectric, or other feature ofhigh-performance integrated circuits.

Dielectric 104-2 comprises any of the materials listed above fordielectric layer 104. In some embodiments, layer 104-2 and layer 104comprise the same material, while in some other embodiments, they aredifferent materials. In some embodiments, layer 104-2 has a thickness of50 nm; but in some alternative embodiments, it has a thickness otherthan 50 nm.

In some alternative embodiments, a composition having a periodicarrangement of layers is formed. One such composition, composition 100D,is depicted in FIG. 4. As shown in FIG. 4, the periodic arrangementincludes alternating layers of dielectric and active material (e.g.,104/106/104-2/106-2 . . . ). This periodic arrangement can be continuedas desired, to create any of a variety of devices of varying complexity,including multiple-quantum-well structures, super lattices, double-gatestructures, and the like. Those skilled in the art, in view of thepresent disclosure, will know how to design, make and use suchstructures.

As described later in this specification (see, e.g., Section V.B.1) anddepicted in FIGS. 8A through 8D, in some embodiments, one or moretransitional layers are present beneath dielectric 104. These layersenable the growth of single-phase rare-earth dielectric material. Incompositions 100A through 100D, these layers, to the extent that theyare present, are included in substrate 102.

Similarly, as depicted in FIGS. 9A and 9B, in some embodiments, one ormore transitional layers are present beneath active layer 106. Theselayers enable the growth of single-phase active layer material. Incompositions 100B through 100D, these layers, to the extent they arepresent, are not shown.

IV. CRYSTAL STRUCTURE OF RARE-EARTH DIELECTRICS

As compared to other high-K dielectric films, single-phase rare-earthdielectric layers provide several key advantages regarding their use inintegrated circuit devices. Specifically, these films enable:

(i) thicker gate layers and buried dielectric layers;

(ii) semiconductor-on-insulator structures with buried dielectric andactive layers that do not exhibit a growth-thickness limitation;

(iii) low thermionic emission of electrons across thedielectric/semiconductor interface;

(iv) semiconductor/dielectric interfaces that exhibit a quality anddefect density which rivals or surpasses that of silicon dioxide onsilicon; and

(v) fabrication of semiconductor-on-insulator structures that comprise asingle-crystal semiconductor layer with a thickness of 100 nanometers orless.

Dielectric films that incorporate rare-earth metals are potentially ameans for providing high-K dielectric films. The term “potentially” isused because there are several important caveats to the use ofrare-earth metals. Specifically, the crystal structure of rare-earthdielectrics can vary significantly, and the crystal structure, in part,makes many of these rare earth dielectrics inappropriate for use inhigh-performance integrated circuits. Furthermore, the crystal structureof a rare-earth dielectric can affect the quality of epitaxially-grownfilms that are deposited on top of the rare-earth dielectric. Forexample, buried dielectric 104 must have high interface quality and asingle-phase morphology to enable the formation of fully-depletedelectrical devices in active layer 106. Rare-earth dielectrics depositedusing methods that are known in the prior art are ill-suited to theformation of fully-depleted SOI devices.

Rare-earth oxides are known to exhibit fluorite-type structures. Thesestructures exhibit morphology differences as a function of the atomicweight of the rare-earth cation present in the oxide, among any otherfactors.

In particular, oxides comprising lighter rare-earths form cubicCaF.sub.2-type crystal structure as a result of possible ionizationstates of +2 and/or +3 and/or +4. Oxides having this crystal structureexhibit significant net charge defect due to a multiplicity of possibleoxidation states (for rare-earth oxides). This renders these rare-earthoxides inapplicable to high-performance field-effect-transistor (FET)devices. These oxides are not suitable for use in conjunction with thevarious embodiments of the present invention.

The layer thickness of rare-earth dielectrics is limited when grown viaprior-art methods. In general, this limitation arises from latticemismatch, internal strain, and/or electronic or structural instabilityof the crystal structure of the rare-earth oxides. Annealing rare-earthoxides that are formed via prior-art methods in order to reduce strainresults in mixed crystal phases (i.e., polycrystalline or amorphous).Layer thickness far exceeding that achieved in the prior art can beattained for rare-earth dielectrics as disclosed herein.

On the other hand, oxides formed from heavier rare-earths (e.g.,RE.sub.2O.sub.3, etc.), exhibit a distorted CaF.sub.2-type crystalstructure which includes anion vacancies due to an ionization state ofRE.sup.3+. The crystal structure associated with rare-earth oxides ofheavier rare earths is also known as “Bixbyite.” These oxides aredesirable for use as dielectric layer 104 in the compositions describedherein.

FIG. 5 depicts the crystal structure diagram of unit cell 500 of arare-earth oxide having the formula RE.sub.2O.sub.3, which in theillustrative embodiment is Er.sup.+3.sub.2O.sub.3. The crystal structureof unit cell 500 is an oxygen-vacancy-derived fluorite derivative (i.e.,Bixbyite structure). Dielectric layers 104, 104-2, etc., (see, e.g.,compositions 100A-100D) comprise an assemblage of these unit cells.

The number and position of the anion vacancies determines the crystalshape of the RE.sub.2O.sub.3 unit cell. The crystal shape of this cellcan be engineered to provide a suitable match to the lattice constant ofthe underlying semiconductor substrate. Oxygen vacancies along the bodydiagonal and/or the face diagonal lead to a C-type cubic structure aswill be discussed below and with reference to FIG. 6. For example, twoanion vacancies per fluorite unit cell causes the unit cell ofEr.sup.3+.sub.2O.sub.3 to increase to nearly twice the unit cell size ofSi. This, in turn, enables low-strain, single-phaseEr.sup.3+.sub.2O.sub.3 to be epitaxially grown directly on a siliconsubstrate.

Furthermore, the number and position of the anion vacancies can beengineered to induce a desired strain (tensile or compressive) in thedielectric layer and/or overgrown layers. For example, in someembodiments, strain in the semiconductor layer is desired in order toaffect carrier mobility.

Each fluorite unit cell has two oxygen vacancies, which lie along thebody diagonal as shown. The presence of these two oxygen vacanciescauses the Er.sup.3+.sub.2O.sub.3 unit cell to double in size, therebydoubling its lattice constant, which provides a suitable match to thelattice constant of <100> silicon.

In some alternative embodiments, oxygen vacancies lie at the ends of theface diagonal. In some other alternative embodiments, oxygen vacanciesare distributed between the ends of the face diagonal and the bodydiagonal.

V. CONSIDERATIONS FOR PRODUCING A SINGLE-PHASE RARE-EARTH DIELECTRIC

Certain factors must be addressed to produce a composition that includesa dielectric layer comprising a single-phase rare-earth dielectric. Inparticular:

(1) rare-earth metals having an atomic number of 65 or less, such ascerium, promethium, or lanthanum, form cations with radii larger than0.93 angstroms, which is unsuitable for use in embodiments of thepresent invention; and

(2) the growth of a polar rare-earth oxide (which comprises cations andanions) on a non-polar substrate (such as silicon or germanium) tendstoward multi-domain growth due to the lack of an energetically-favorablebonding site for one of either the cations or anions of the rare-earthdielectric.

A. Cation-Radius Dependency of Rare-Earth Dielectrics

The uniformity and stability of the crystal structure of a rare-earthoxide is dependent upon the radius of the included rare-earth cation.FIG. 6 depicts a chart of the polymorphs of rare-earth oxides versustemperature and as a function of cation radius.

Regions A through C are regions of temperature and cation radius whereinthe crystal structure of the polymorphs of rare-earth oxides areunstable and are not limited to a single type over all temperatures.Therefore, rare-earth oxides formed using these rare-earth elements willexhibit polycrystalline or multi-domain crystal structure. Such oxidesare undesirable for use in conjunction with the compositions that aredisclosed herein.

For example, the crystal structure of a rare-earth oxide comprisinglanthanum, which has a cation radius of 1.14, changes as the temperatureof the crystal reduces from growth temperature to room temperature. Thecrystal structure of such a lanthanum-oxide will change from an A-typehexagonal structure above 400.degree. C. to a C-type metastablestructure below 400.degree. C.

Region D is the only region wherein the rare-earth oxide polymorphs arestable over the temperature range from room temperature to 2000.degree.C. The rare-earth oxide polymorphs that exist in region C includesesquioxides that have a cation radius less than 0.93. The rare-earthelements that have cation radii less than 0.93 include dysprosium,holmium, erbium, thulium, ytterbium, and lutetium. These rare-earthelements are also characterized by an atomic number greater than orequal to 66. These rare-earth metals, therefore, will form a stableoxygen-vacancy-derived fluorite crystal structure (i.e., bixbyite) thatexhibits single-phase structure. Consequently, rare-earth metals thatare suitable for use in conjunction with the illustrative embodimentinclude dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

B. Avoidance of Multi-Domain Growth of a Rare-Earth Dielectrics

FIG. 7A depicts the crystal structure of prior-art composition 700,which includes substrate 702 and rare-earth oxide 704.

Substrate 702 is a silicon substrate having a <100> crystal orientation.The substrate comprises a homogeneous distribution of silicon atoms 712.Silicon (and germanium) wafers have non-polar substrate surfaces sincethe unit cell of silicon (or germanium) crystals exhibit no polarity.

Rare-earth oxide 704 is a multi-domain, single-crystal layer that hastwo domains of single-crystal structure, Domain A and Domain B. Thesedomains are each single-crystalline and have the same basic crystalstructure, which is represented by unit cells 706.

Unit cell 706 comprises an arrangement of rare-earth cations 708 andoxygen anions 710, which are arranged asymmetrically. This asymmetryleads to a polarity for unit cell 706. The polarity of unit cell 706 inDomain A is opposite that of the polarity of unit cell 706 in Domain B.Since substrate 702 is a non-polar substrate, there is noenergetically-favorable bonding site for either the rare-earth cations708 or the oxygen anions 710. Therefore, crystal growth of rare-earthoxide 704 can begin in any region of the substrate with the depositionof either cations or anions. The multi-domain nature of rare-earth oxide704 leads to a net-charge defect density that is too high for use inhigh-performance integrated circuits that are formed in a semiconductorlayer disposed on rare-earth oxide 704. In addition, the multi-domainnature of rare-earth oxide 704 leads to undesirable surface roughnessand/or structural non-uniformity.

During growth of rare-earth oxide 704, growth of the crystal structureof Domain A initiates on Locality A of substrate 702. At the same time,growth of the crystal structure of Domain B initiates on Locality B ofsubstrate 702. Since the crystal structure initiates on the two regionsof the non-polar substrate independently, and no energetically-favorableanion or cation bonding sites exist, nothing ensures that theorientation of the unit cells in Locality A and Locality B are the same.As a consequence, the polarity of unit cell 706 is oriented differentlyin Domain A and Domain B, and there exists an antiphase domain boundarybetween Domain A and Domain B. Simply put, rare-earth oxide 704 is not asingle-phase material and, as such, is not suitable for use as layer 104of composition 100A.

FIG. 7B depicts a crystal structure diagram of a specific embodiment ofcomposition 100A (see FIG. 1, etc.). As previously described,composition 100A comprises single-phase rare-earth dielectric 104disposed on a substrate 102.

In the embodiment that is depicted in FIG. 7B, substrate 102 is asilicon wafer having a <100> crystal orientation and a homogeneousdistribution of (silicon) atoms 712. As previously disclosed, in somealternative embodiments, substrate 102 comprises a material other than<100> silicon, such as a germanium wafer having a <100> crystalorientation and a homogeneous distribution of (germanium) atoms, or asilicon or germanium wafer that has a crystal orientation other than<100>, such as <111>, etc.

In the embodiment that is depicted in FIG. 7B, single-phase dielectric104 is a rare-earth oxide, such as a single-phase layer ofEr.sup.3+.sub.2O.sub.3, as represented by unit cells 714. In contrast tounit cell 706 of rare-earth oxide 704, the polarity of unit cell 714 isuniform throughout the rare-earth oxide. That is, the rare-earth oxideexhibits single domain morphology.

In the embodiment shown in FIG. 7B, monolayer 716 of oxygen anions 710is first deposited on the surface of substrate 102, thereby forminganion-terminated surface 718. Anion-terminated surface 718 providesenergetically-favorable bonding sites for rare-earth cations 708, thusensuring uniform orientation of unit cells 714.

1. Surfaces Suitable for Growing Single-Phase Rare-Earth Dielectrics

Rare-earth dielectrics typically comprise polar crystals. In order toform single-phase rare-earth dielectrics on a non-polar substrate, suchas a silicon wafer, the substrate surface must provideenergetically-favorable bonding sites for one of either anions orcations. Several methods have been found to provide a substrate surfacethat enables epitaxial growth of single-phase rare-earth dielectricsthat have bixbyite oxygen-vacancy-ordered crystal structure. Thesemethods are listed below; structures formed by the methods are depictedin FIGS. 8A through 8D:

(i) providing an anion or cation terminated surface;

(ii) compensation of the semiconductor surface using an oxide ornitride;

(iii) preferential growth on miscut semiconductor wafers;

(iv) compensation of the semiconductor surface with material having arock-salt crystal structure having a non-polar surface, such as the<001> surface of ytterbium-monoxide (YbO) or the <001> surface of erbiumnitride; and

(v) anion ordering using a short-period superlattice. It is notable thatcompositions 800A through 800D depicted in respective FIGS. 8A through8D are consistent with composition 100A of FIG. 1. Compositions 800Athrough 800D simply show further details of various transitional layers,etc., that enable composition 100A.

FIG. 8A depicts composition 800A in accordance with a first embodimentand based on methods (i) or (ii), above. Composition 800 comprises wafer802, seed layer 804, and rare-earth dielectric 104. Wafer 802 and seedlayer 804 collectively represent substrate 102 of composition 100A ofFIG. 1. As described further below, seed layer 804 providesenergetically-favorable bonding sites for rare-earth oxide anions (orcations), according to the illustrative embodiment of the presentinvention.

In some embodiments, wafer 802 comprises silicon having a <100> crystalorientation.

Rare-earth dielectric 104 has been described earlier; in thisembodiment, it is assumed to be a layer of erbium oxide having theformula Er.sup.3+.sub.2O.sub.3 and a thickness of about 5 microns.Rare-earth dielectric 104 is epitaxially-deposited on seed layer 804 byatomic-layer epitaxy (“ALE”).

In some embodiments, seed layer 804 provides an energetically-favorablebonding surface for cations that are present in rare-earth dielectric104 by terminating the non-polar silicon bonds with anions (see, e.g.,anion-terminated surface 718 of FIG. 7B). Thus, the combination of wafer802 and seed layer 804 serves as the substrate (i.e., substrate 102)upon which the rare-earth dielectric 104 is grown. During ALE, thesurface mobility of the rare-earth cations promotes the growth of therare-earth dielectric in the same manner across the entire surface ofthe substrate, thereby forming a single-phase crystalline layer.

In some embodiments in which seed layer 804 provides an anion-terminatedsurface, the seed layer is one or more monolayers of oxygen atoms. Theoxygen atoms terminate the semiconductor (e.g., silicon) bonds that areavailable at the surface of wafer 802. Seed layer 804 isepitaxially-deposited on the surface of wafer 802 by ALE.

In some embodiments that provide an anion-terminated surface, seed layer804 is silicon dioxide, silicon nitride, or silicon oxynitride. Sincesilicon dioxide, silicon nitride, and silicon oxynitride are amorphous,they should be present as no more than a few monolayers. Althoughamorphous in thicker layers, these materials will retain an underlyingcrystal structure in layers of only a few monolayers. In still otheralternative embodiments, seed layer 804 comprises a rare-earth nitridelayer, such as erbium nitride. In yet some embodiments, seed layer 804comprises a rare-earth phosphide.

In some embodiments, seed layer 804 provides an energetically-favorablebonding surface for anions that are present in rare-earth dielectric 104by terminating the non-polar semiconductor (e.g., silicon, etc.) bondswith cations. During ALE, the surface mobility of the rare-earth anionspromotes the growth of the rare-earth dielectric in the same manneracross the entire surface of the substrate, thereby forming asingle-phase crystalline layer.

In some embodiments that provides a cation-terminated surface, seedlayer 804 is one or more monolayers of rare-earth cations, such as arare-earth metal (e.g., Er.sup.+3, etc.).

FIG. 8B depicts composition 800B, which includes wafer 802 andrare-earth dielectric 104. In accordance with a second embodiment andbased on method (iii) listed, wafer 802 is miscut, which enablessingle-phase crystalline growth of rare-earth dielectric 104 directly onthe wafer without the use of a transitional layer. For composition 800B,wafer 802 is equivalent to substrate 102 of composition 100A.

In some embodiments, wafer 802 comprises a miscut <100> silicon wafer,wherein the crystalline orientation of the wafer is cut at an angle ofabout 6.degree. from the <100> surface toward the <110> direction. Insome other embodiments, wafer 802 is a miscut germanium wafer.

FIG. 8C depicts composition 800C comprising wafer 802, polar layer 806,and rare-earth dielectric 104.

In some embodiments, polar layer 806 is ytterbium monoxide (YbO), whichenables single-phase crystalline growth of a rare-earth dielectric.Polar layer 806, which is several monolayers thick, is deposited onnon-polar wafer 802 using ALE, thereby effectively changing thecharacter of the substrate surface from non-polar to polar. In thiscomposition, the combination of wafer 802 and polar layer 806 serve assubstrate 102 of composition 100A. In some other embodiments, polarlayer 806 is erbium nitride (ErN).

YbO and ErN are different from most of the rare-earth dielectricspreviously described to the extent that it forms the crystal structureof rock-salt, rather than bixbyite. The crystal structure of rock-saltis well-known to those skilled in the art. Depending upon growthconditions, a layer of YbO can form either a polar or non-polar surface.For example, the <001> surface of YbO has anions and cations in the sameplane, thereby providing a non-polar surface. Therefore, YbO can be usedto change the surface state of a non-polar surface to polar or,alternatively, to change the surface of a polar surface to non-polar. Inaddition, YbO has a lattice constant that is closely-matched to thelattice constant of silicon. Therefore, YbO provides a low-strain meansof terminating the non-polar silicon surface and providing a polarsurface that is favorable for epitaxial growth of a single-phaserare-earth dielectric, such as Er.sup.3+.sub.2O.sub.3.

In some alternative embodiments, rare-earth dielectric 104 comprises alayer of YbO, such that rare-earth dielectric 104 is simply acontinuation of polar layer 806.

FIG. 8D depicts composition 800D, which includes wafer 802,super-lattice 808, and rare-earth dielectric 104. Superlattice 808enables the growth of a single-phase rare-earth dielectric. Incomposition 800D, wafer 802 and superlattice 808 represent substrate 102of composition 100A.

In some embodiments, superlattice 808 comprises alternating layers ofoxygen-rich erbium oxide and oxygen-poor erbium oxide having respectiveformulas ErO.sub.1.5+y and ErO.sub.1.5+x, where x and y are less thanone. The layer structure of the superlattice enables (i) the strain dueto lattice mismatch and (ii) the anion ordering to be engineered.

In some alternative embodiments, superlattice 808 comprises anotheranion-rich/cation-rich superlattice structure, such as ytterbium oxideand erbium oxide. In some further embodiments, super lattice 808comprises alternating layers of erbium oxide and erbium nitride as ameans of ordering the bixbyite oxygen vacancies.

VI. ACTIVE-LAYER EPITAXY ON RARE-EARTH DIELECTRICS

In order to form semiconductor-on-insulator structure 100B that isdepicted in FIG. 2, such that it is suitable for high-performance FETdevices, active layer 106 must have a single-phase crystal structure.The optimal deposition surface for silicon (or germanium) epitaxy isnon-polar, since silicon, germanium, and silicon-germanium are non-polarcrystals. Since most rare-earth dielectrics typically comprise polarcrystals, epitaxial-growth of silicon or germanium on a rare-earthdielectric has proven problematic. Stress at the interface andlattice-constant mismatch are two critical issues that must be addressedin order to achieve single-phase active layer on a rare-earthdielectric.

Several methods have been found to provide a rare-earth dielectricsurface that enables epitaxial growth of single-phase non-polarsemiconductors, such as silicon and germanium. Compositions that areformed using these methods are depicted in FIGS. 9A and 9B.

FIG. 9A depicts composition 900A in accordance with an embodiment of thepresent invention. Composition 900A comprises substrate 102, dielectric104, interface layer 902, and active layer 106. Interface layer 902provides a suitable surface for epitaxial growth of a non-polarsemiconductor (to form active layer 106) on dielectric layer 104. It isnotable that composition 900A (and composition 900B of FIG. 9B) isconsistent with composition 100B of FIG. 2. Compositions 900A and 900Bsimply show further various transitional layers, etc., that enablecomposition 100B.

Substrate 102, dielectric 104, and active layer 106 have been describedpreviously. It is understood that substrate 102 includes transitionallayers, to the extent that they are present, as depicted in FIGS. 8A,8C, and 8D.

In some embodiments, dielectric 104 is a layer of rare-earth oxide suchas erbium oxide, Er.sup.3+.sub.2O.sub.3 and has thickness of about 1000nanometers. In some embodiments, active layer 106 is a layer ofepitaxially-grown single-phase silicon that has a thickness of about 100nm.

In some embodiments, interface layer 902 is a rare-earth nitride, suchas erbium nitride. Erbium nitride has a rock-salt crystal structure thatcan have a surface character that is either polar or non-polar. Erbiumnitride, therefore, can provide a change in the character of the growthsurface for active layer 106 from polar (i.e., the surface character oferbium oxide) to non-polar. In some embodiments in which erbium nitrideis used for interface layer 902, the thickness of that layer is lessthan five monolayers.

In addition to changing the character of the growth surface to enabledeposition of active layer 106, rare-earth nitrides, such as erbiumnitride, also provide a diffusion barrier to boron. This is importantsince boron can be diffused or implanted into active layer 106 to formtransistors, etc. In the absence of a suitable diffusion barrier, boronthat was implanted in active layer 106 can diffuse into dielectric layer104. The presence of boron in dielectric layer 104 has deleteriouseffects on the performance of composition 100B when used for thefabrication of high-performance integrated circuits and devices.

In some alternative embodiments of the present invention, interfacelayer 902 comprises a layer of YbO. Like erbium nitride, YbO has arock-salt crystal structure, which can provide a surface character thatis either polar or non-polar. YbO, therefore, can also provide a changein the character of the growth surface for active layer 106 from polar(i.e., the surface character of erbium oxide) to non-polar. In stillsome other alternative embodiments of the present invention, dielectriclayer 104 comprises YbO, such that interface layer 902 is a continuationof dielectric layer 104.

FIG. 9B depicts composition 900B, in accordance with an embodiment ofthe present invention. Composition 900B comprises substrate 102,single-phase dielectric layer 104, termination layer 904, nucleationlayer 906, and active layer 106.

Termination layer 904 and nucleation layer 906 serve as an interfacethat provides a suitable surface for epitaxial growth of a non-polarsemiconductor on a rare-earth dielectric, according to an alternativeembodiment of the present invention.

In some embodiments, termination layer 904 comprises one or moremonolayers of rare-earth metal, such as erbium. Termination layer 904provides a wetting surface suitable for the subsequent growth ofnucleation layer 906. In some alternative embodiments, termination layer904 comprises a rare-earth nitride, such as erbium nitride. In somefurther alternative embodiments, termination layer 904 comprises anoxygen terminated surface.

In some embodiments, nucleation layer 906 comprises one or moremonolayers of low-temperature-growth silicon. Nucleation layer 906provides nucleation sites for the subsequent epitaxial growth of activelayer 106.

While nucleation layer 906 is beneficial for the epitaxial deposition ofsilicon on termination layer 904, in some alternative embodiments of thepresent invention wherein active layer 106 comprises germanium,nucleation layer 906 is optional.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. For example, in thisSpecification, numerous specific details are provided in order toprovide a thorough description and understanding of the illustrativeembodiments of the present invention. Those skilled in the art willrecognize, however, that the invention can be practiced without one ormore of those details, or with other methods, materials, components,etc.

Furthermore, in some instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the illustrative embodiments. It is understood that thevarious embodiments shown in the Figures are illustrative, and are notnecessarily drawn to scale. Reference throughout the specification to“one embodiment” or “an embodiment” or “some embodiments” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment(s) is included in at least one embodimentof the present invention, but not necessarily all embodiments.Consequently, the appearances of the phrase “in one embodiment,” “in anembodiment,” or “in some embodiments” in various places throughout theSpecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, materials, orcharacteristics can be combined in any suitable manner in one or moreembodiments. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

1. A composition comprising: a first layer comprising a firstrare-earth; a second layer comprising a second and third rare-earth; athird layer comprising a fourth rare-earth; and a fourth layercomprising a Group IV semiconductor wherein the second layer separatesthe third layer from the first layer and the third layer separates thefourth layer from the second layer such that the third rare earth isdifferent from the first rare earth and the first, second and fourthrare earth may be the same.
 2. The composition of claim 1 furthercomprising a fifth layer comprising a fourth rare earth and a sixthlayer comprising a Group IV semiconductor wherein the fifth layerseparates the sixth layer from the fourth layer.
 3. The structure ofclaim 2 wherein said fifth layer comprises at least two layers with atleast two different rare earths in one of the at least two layers. 4.The structure of claim 1 wherein at least one of said first, second,third and fourth rare earth is chosen from a group consisting of erbium,ytterbium, dysprosium, holmium, thulium, and lutetium.
 5. The structureof claim 1 wherein at least one of said first, second, third and fourthrare earths is a rare-earth of atomic number greater than or equal to66.
 6. The composition of claim 1 wherein said fourth layer comprises amaterial selected from the group consisting of silicon, germanium,silicon-germanium, gallium arsenide, indium phosphide, and siliconcarbide.
 7. The composition of claim 2 wherein said sixth layercomprises a material selected from the group consisting of silicon,germanium, silicon-germanium, gallium arsenide, indium phosphide, andsilicon carbide.
 8. The composition of claim 2 wherein at least one ofsaid fourth layer and sixth layer are a quantum dot structure.
 9. Thecomposition of claim 1 further comprising a seed layer such that thefirst layer is between the seed layer and the second layer.
 10. Thecomposition of claim 2 further comprising a seed layer such that theseed layer is between the fourth layer and the fifth layer.
 11. Thecomposition of claim 1 wherein said third layer further comprises atermination layer and a nucleation layer.